Why is the demographic transition an important yet difficult problem for behavioral ecologists?

Demographic transition is an important yet difficult problem for behavioural scientists because they are baffled as to why people would start limiting their reproduction in the 19th century when it was said that resources were plentiful at the peak of the Industrial Revolution. Just when there was apparently more than enough resources, people start to lower their reproductive activity which goes against all reasoning. Behavioural ecologists cannot seem to pinpoint the exact reason for why high birth and death rates transition to lower birth and death rates when a country transitions from a pre-industrialized economic system to an industrialized economic system.